Very large-scale integrated microelectronic circuits consist of a plurality of semiconducting elements which are produced by controlled doping and structuring of monocrystalline silicon. These individual semiconductor elements are connected to form a functioning unit by a layer structure, the so-called interconnect, which consists of conductive tracks and the interlayers needed for insulation.
Progressive miniaturisation is placing extreme requirements on the materials that are used. Besides the semiconducting transistors, the properties of the interconnect determine the performance features of these very large-scale integrated microelectronic circuits. These requirements are dictated by the ever-higher clock frequencies and shorter signal propagation times needed for this.
In this case, a high conductance of the conductive-track material and a low dielectric constant of the insulator material are desirable. The miniaturisation of the semiconductor elements and of the interconnect detrimentally affect the component properties. Reducing the conductive-track cross sections increases the resistance of the conductive tracks. The smaller spacing between the conductive tracks, which is filled with insulator material, leads to an electrical interaction between the various conductive tracks and therefore to undesired signal lags. The interactions between the conductive tracks depend to a great extent on the relative permittivity ∈ (the k value) of the insulator material.
Attempts are being made to counteract these technical difficulties by using conductive materials with a higher conductivity and insulator materials with a lower dielectric constant. For instance, the aluminium previously used as a conductive-track material is being successively replaced by copper, which has a higher conductivity.
Silicon dioxide has to date proved useful, in terms of both its electrical properties and its process properties, as an insulator material in the production of very large-scale integrated circuits. The dielectric constant of silicon dioxide is about 4.0. For modern requirements, however, this value is too high. For new chip generations, dielectric materials with k values significantly lower than 3.0, preferably lower than 2.5, are needed.
The value of the relative permittivity (k value) depends strongly on the temperature at which this value is determined. The values indicated here should be understood to be the values obtained when determined at 22° C. and a pressure of 1 bar.
Besides the dielectric constant (the k value), a number of other properties need to be taken into account for the integration of a new material in a semiconductor process as a replacement for silicon dioxide. For example, the dielectric material must be able to withstand high process temperatures of up to 400° C., which are reached during subsequent metallisation and annealing steps. It is furthermore necessary for the layer materials, or their precursors, to be available at a sufficient purity, since impurities, in particular metals, can detrimentally affect the electrical properties of the layer materials.
The dielectric material should be as easy as possible to work with, and it should be possible to apply it as a thin layer by using a standard industrial method, for example, the spin coating method.
Besides organic polymers, silsesquioxanes and carbon-doped silicon dioxide (SiOC) are described as dielectrics with a dielectric constant lower than 3.0.
Organic polymers, as dielectrics with a low dielectric constant, have gained acceptance in technical fabrication. The properties of these polymers, however, lead to significant problems in process integration. For instance, their limited chemical and mechanical stability at elevated temperatures restricts the subsequent process steps. Requisite polishing steps, for example, are optimised for layers that resemble silicon dioxide, and they often do not lead to optimum results on organic polymer layers.
Silsesquioxanes are siliconorganic polymers which are applied as oligomer solutions in spin coating methods and are then thermally crosslinked. WO 98/47944 A1 teaches the use of organosilsesquioxanes to produce layers with k values lower than 2.7. These compounds, however, are obtainable only via elaborate synthesis routes from trialkoxysilanene. U.S. Pat. No. 5,906,859 teaches the application of oligomeric hydridosilsesquioxanes, which are thermally crosslinked to form polymers. Dielectric constants of 2.7–2.9 are achieved with the compounds described in U.S. Pat. No. 5,906,859.
Carbon-doped silicon dioxide is applied from organosilanes in a PE-CVD method (Plasma Enhanced Chemical Vapour Deposition) with a reactive oxygen plasma.
Carbon-doped silicon dioxide, because of its silicon dioxide matrix, has similar process properties to silicon dioxide, and it is therefore much easier to integrate in the production process. The dielectric constant of these layers is lowered in relation to silicon dioxide by the carbon content. U.S. Pat. No. 6,054,206 teaches the application of such layers from gaseous organosiloxanes. The high-vacuum plasma CVD process, however, is elaborate and entails high costs. Dielectric constants of 2.7–2.9 are likewise achieved with carbon-doped silicon dioxide layers.
WO 99/55526 A1 also describes the production of dielectric layers by means of a CVD process, preferably a plasma CVD process. Starting from organosilicon precursors, layers are obtained which have a backbone structure of Si—O—Si bonds, with organic side groups being bonded to this structure. The CVD process is preferably carried out in such a way that the backbone has ring-like structures. In particular, cyclic organosiloxanes are suitable as precursors. The layers produced according to the examples have dielectric constants between 2.6 and 3.3.
WO 00/75975 A2 teaches the use of polycarbosilanes, which are applied from a solution and are converted into polyorganosilicon layers with k values lower than 2.5 by heat treatment in discrete steps. The polycarbosilanes that are used are hydridopolycarbosilanes, which contain at least one hydrogen atom bonded to silicon, as well as preferably allyl substituents. Si—H bonds, however, are moisture-sensitive and therefore need to be handled accordingly. The platinum compounds that are used to crosslink Si—H compounds with unsaturated groups are undesirable as metallic impurities. In order to obtain layers with low k values, the heat treatment needs to be carried out under accurately controlled conditions, with the need to comply with various fixed temperature steps.
A more general way of further reducing the dielectric constant, i.e., the k value, of dielectric materials is to introduce pores. The air contained in the pores has a k value of close to 1. If air-filled pores are introduced into a dense material, then the average k value of the material is a combination of the k value of the dense material and partially the k value of air. A reduction of the effective k value is hence achieved. The k value of pure silicon dioxide can in this way be reduced from 4.0 to lower than 2.0, although porosities >90% are needed for this (L. Hrubesch, Mat. Res. Soc. Symp. Proc., 381, (1995), 267). Such a high porosity lowers the mechanical stability and compromises the processability of these layers considerably.
The principle is generally applicable to dense dielectric layers. For layers with lower initial k values, it is possible to achieve k values lower than 2.0 with substantially smaller porosities, which in turn benefits the mechanical stability of the layers.
There is, however, a lack of readily obtainable starting materials that are suitable for the production of dielectric layers in a simple thermal method. The German Patent Specification DE 196 03 241 C1 describes the production of multifunctional organosiloxanes, which are used as crosslinkers in inorganic surface coating agents based on silica sol. After drying, these materials form soft films that are ill-suited as dielectric layer materials and have k values significantly higher than 3.
It has now been found, unexpectedly, that it is also possible to produce dielectric layers with low k values by thermal treatment from sol-gel products of multifunctional carbosilanes, for example compounds which are known from DE 196 03 241 C1. In particular, it is in this case possible to use carbosilanes that do not have any Si—H bonds. This is surprising, in particular, against the background of the teaching of WO 00/75975 A2, which states on page 8, last paragraph, that only polycarbosilanes that contain at least one hydrogen atom bonded to silicon are suitable for the production of corresponding dielectric layers.
After the thermal treatment, the dielectric layers according to the invention resemble carbon-doped silicon dioxide in terms of their composition, and they combine low k values with the advantage of a simple heat treatment.